This invention provides a multistage catalytic reactor of unitized internal construction for dividing the reactor into a plurality of catalytic reaction stages and support within each stage of one or more beds of catalyst particles.
In the sulfuric acid industry and in other industries using large catalytic reactors which must be constructed at the plant site, present commercial construction of the reactor internals is hazardous and expensive. Typically at the present time, in large sulfuric acid plant reactors for catalytic oxidation of sulfur dioxide to sulfur trioxide the internal division plates separating the reactor into the several catalytic stages and the catalyst support grids in each stage are supported on a plurality of columns within the reactor in a more or less unfastened, stacked manner so that the resulting internal structure is not unitized and can be quite unsteady, particularly in the larger diameter reactors.
Reactor design favors the use of the largest diameter reactor possible so that the catalyst bed in each stage can be as shallow as possible offering lower pressure drop as the gas or other fluid passes through the catalyst bed and better conversion efficiency. With plants being designed today for production of 1,000 and more metric tons per day of sulfuric acid, reactors of 40 to 50 feet (about 12 to 15 meters) in diameter are not uncommon. At such large diameters, collapse of the reactor internals during construction or during operation is of serious concern, requiring special precautions and increased costs. Severe injury and death is an ever-present hazard and occasionally occur.
FIGS. 2 and 3 of the drawings show one typical such construction presently used. Only a portion of a reactor is shown in these figures for a brief discussion of the problem. Vessel 1 contains division plates 2 (one shown) and catalyst support grids 3 (one shown) which merely rest on horizontal flanges 4 of vertical support columns 5, and at the periphery of vessel 1 they rest on an annular flange 6. FIG. 3 shows a top view of how the catalyst support grid 3 is formed by a plurality of triangularly shaped perforated plates 7 (perforations not shown) supported at each appex by a horizontal flange 4 of a vertical support column 5. In this typical type of present construction, to maximize structural stability as much as non-unitized construction will permit the vertical support columns 5 are usually not spaced more than on 3 to 4 feet (about 1 to 1.3 meters) apart. Loading of catalyst into each reactor stage with the equipment which has been developed is rather difficult with such closely spaced vertical columns.
Such construction techniques were developed to enable use of a construction which allows for thermal expansion and contraction of the internals between atmospheric temperature and the high operating temperatures attained in a catalytic reactor, particularly with exothermic reactions.
A fully loaded operating reactor develops tremendous internal forces. Internal weights of 1.5 million pounds (about 3.3 million kilograms) of catalyst, catalyst support screens and grids, division plates and vertical support columns are not uncommon. The force of this weight plus the downward force of the pressure drop through the several catalyst beds places the internal construction under very high compression which is transmitted to the bottom of the vessel through the vertical support columns 5 as shown by the downward arrow in FIG. 2. These downward forces are equally balanced by a very high upward tension force in the outer circumference of the vessel from its base to its top as shown by the upward arrow in FIG. 2, causing deflection of the base of the vessel 1 around its circumference as shown in FIG. 4, unless the base is securely fastened to a massive concrete foundation by bolts strong enough to minimize or prevent such deflection. Any significant deflection which does occur will of course contribute to instability of the non-unitized internal structure and its possible collapse. Yet tie-downs sufficiently strong to prevent or reduce the deflection which would otherwise occur requires an extremely massive and strong (in tension) foundation and tie-down system.
Another disadvantage of current construction of catalytic reactors is that such non-unitized construction techniques require that the first stage of a multistage catalytic reactor be at the top of the vessel. Moreover, to better resist the forces against the top of the vessel, the top normally is made with a domed shape, which is more expensive than a flat-top would be.
Typically, the gas or other fluid stream being processed is charged into the first catalytic stage at sufficient pressure to provide for the entire pressure drop through the entire subsequent process. This means sufficient pressure to provide for the pressure drop through the several beds of catalyst (e.g., usually 3 or 4 catalyst stages), plus through the equipment the fluid is caused to flow through between each stage. For example, in an exothermic reaction such as oxidation of sulfur dioxide, the gas stream is cooled between each catalytic stage by flowing through one or more gas-to-gas heat exchangers or through waste heat boilers, and in some processes also through an intermediate absorption tower for recovery of sulfur trioxide between two of the catalytic stages. Thus, the first catalytic stage is at substantially higher pressure than the second stage, which in turn is at higher pressure than the third stage, and so on.
To accommodate this with a non-unitized internal construction requires that the highest pressure stage (i.e., the first stage) be at the top of the reactor, with the next highest pressure stage (i.e., the second stage) just below the first stage, and so on; otherwise, if a higher pressure stage were located below a lower pressure stage, the pressure differential could cause lifting of the division plate between the two stages causing leakage of the gas or other fluid past the division plate directly into the lower pressure stage and possible collapse of the internals. Yet for several reasons, it is not advantageous to have the first catalytic stage at the top of the reactor. Any dusts such as fly ash, etc. in a gas stream charged to the catalytic reactor are removed by the catalyst bed in the first stage where the catalyst bed acts as a filter for such dusts. Thus, the first stage catalyst bed must be removed at relatively frequent intervals for cleaning and then re-installed, yet in a large reactor the first stage may be 40 to 50 feet (12 to 15 meters) above the foundation of the reactor. Also, to minimize pressure drop and to maximize conversion efficiency in the first stage it would be advantageous to make the diameter of the reactor larger at the first stage so as to accommodate the same amount of catalyst with a shallower bed depth, but such construction is not feasible at the top of a vessel. Presently, to accomplish this, it is necessary to use two catalytic reactors, one for the first stage and the second reactor for the subsequent stages. Thus, an internal construction which permits placing the first catalytic stage at the bottom of the reactor would offer both process and cost advantages not presently being concurrently attained.
Thus, for safety and cost, there is a definite need for a unitized internal support construction in multistage catalytic reactors which avoids the danger of collapse, better distributes internal forces so as to eliminate or reduce the propensity for deflection of the bottom of the vessel, provides a more open internal vertical support column structure for easier access of automatic catalyst charging equipment to all regions of the interior of the reactor, permits construction of catalytic reactors of even greater diameter than present without risk of collapse and injury, and/or permits construction of catalytic reactors having the first catalyst stage at the bottom of the vessel.